This invention relates generally to antennas and more particularly to antenna shaping to provide a desired radiation pattern.
It is well known in the antenna art that the shape of an antenna effects both the beam pattern of the antenna and the frequency range over which the antenna operates. (Hereinafter, antennas will be described as transmitting radio frequency energy. However, one skilled in the art will realize that antennas also operate to receive radio frequency energy and the dual of every statement about transmissions holds true for antennas used to receive signals). The shape impacts such operating parameters as: the elevation and azimuthal coverage, which is measured by the directions in space where the antenna transmits signals having levels within 3 dB of the maximum level; the antenna gain; magnitude of the antenna sidelobes; and the amount of ripple in the main beam, which is measured by the amount the gain changes over the elevation and azimuthal coverage areas.
Many antenna parameters are not independent. It is, therefore, not possible to attain arbitrary values for all parameters. For example, increasing the beam coverage area might also increase the sidelobes and ripple. In actual applications, an antenna design is selected which represents a compromise between the various antenna parameters.
One application requiring special antenna design is electronic counter measure (ECM) transmissions. For instance, it is often desirable for an ECM system to transmit signals with a wide azimuthal coverage with low sidelobes and with low ripple. It might also be desirable for an antenna in an ECM system to have relatively high gain.
One antenna sometimes used for such applications is known as a biconical horn antenna. Such antennas have symmetrical upper and lower sections shaped like cones with the tapered ends of the cones facing each other. The cones making up the upper and lower sections are cut along a centerline from base to tip with the cut portions mounted against a ground plane. Signals are coupled to the antenna in one of several ways. A coaxial cable running through the center of one of the sections might have its outer conductor connected to one half of the antenna and its inner conductor connected to the other half of the antenna. Alternatively, a circular waveguide might run through one of the sections and have its opening in the region between the conical sections.
In many applications, the antenna is mounted very near the ground or, if on a ship, near the water. It would be desirable for an antenna to direct radio frequency signals into the regions above the ground or the water without directing any signals into the ground or water. If a biconical horn antenna is tilted upward, energy will be radiated above the ground or water in the regions directly in front of the antenna. However, tilting a biconical horn antenna has little impact on the elevation coverage near the sides of the antenna. The biconical horn antenna is therefore not well suited to applications where asymmetrical elevation coverage is desired.